1. Field of the Invention
This invention relates in general to hydraulic brake systems and in particular to a circulating system for vehicle hydraulic brakes.
2. Description of the Prior Art
Leakage of brake fluid and resulting brake failure are serious problems for vehicles, particularly the hydraulic brakes of aircraft. Aircraft brakes, like those in automobiles and trucks, have a reservoir, master cylinder, and wheel cylinders. Depression of the brake pedal applies pressure from the master cylinder to the wheel cylinder, pressing the brake pads against the wheel drum. As the fluid surges forward, air is drawn into the reservoir from the vent. Moisture from the air condenses, and this water being heavier than hydraulic fluid sinks to the wheel cylinders. The accumulation of water causes pitting of the wheel cylinders and piston, allowing leakage past the O-ring seals. In addition the accumulation of water may freeze causing complete brake failure.
Should a leak develop or the brakes freeze, landing an aircraft can be highly dangerous, particularly on tail wheel planes that rely on brakes for steering. As the brakes are pumped, more air is drawn in through the point of leakage, making the brakes even softer.
Also repairing the brakes in an aircraft or any vehicle is a nuisance. To remove the air from the line, the fluid has to be bled through a port as the brakes are pumped. This is time consuming and often results in wasted fluid.
Others have recognized the disadvantages of conventional noncirculating hydraulic brake systems. In U.S. Pat. No. 3,520,136, a return line from the bleedoff port to the reservoir is provided with a metering device that allows a small amount of fluid to return. Thus the system self-circulates in the normal direction as the brakes are pumped. Should a leak occur, a pressure tank applies pressure to the fluid in the main lines. In that device, however, conventional bleeding would have to be used for repairs, and the emergency system is complex and would not allow selective braking for steering a tail wheel aircraft.